1. Field of the Invention
This invention relates generally to laser manufacturing and, more particularly, to vertical cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
In many applications, it is desirable to have lasers capable of producing light at several different wavelengths that are closely spaced. In particular, for optical communication applications, such as wavelength division multiplexing (WDM), many wavelengths spaced a few nanometers apart are useful. There are several possible ways to make wavelength-selectable arrays with edge-emitting semiconductor lasers. However, edge-emitting semiconductor lasers that meet specifications for telecommunication applications are typically complex and expensive to manufacture. Monolithic edge-emitting semiconductor laser arrays with large numbers of wavelengths are impractical technically and economically.
Vertical cavity surface-emitting lasers (VCSELs) provide a more cost effective solution for many applications. Vertical cavity surface-emitting lasers (VCSELs) can be made using wafer-scale processing and testing, dramatically lowering the cost in comparison to edge-emitting semiconductor lasers, for example. In a vertical cavity surface-emitting laser (VCSEL), the wavelength may be determined by the optical cavity length. The optical cavity length is the effective distance between the two generally parallel mirrors, typically distributed Bragg reflectors (DBRs), enclosing the active region of the vertical cavity surface-emitting laser (VCSEL). Since the optical cavity length is typically set by the epitaxial growth, which should be uniform across the entire wafer or workpiece, the wavelength is uniform.
Tunable vertical cavity surface-emitting lasers (VCSELs) are desired in order to provide different wavelengths on the same wafer or workpiece. One conventional approach to providing tunable vertical cavity surface-emitting lasers (VCSELs) uses a top mirror that is suspended on a micromachined cantilever. With this conventional structure, any given vertical cavity surface-emitting laser (VCSEL) can be tuned to any wavelength within the tuning range. However, this conventional approach involves a micromachined structure that is difficult to fabricate, has reliability problems and is susceptible to mechanical vibrations. A more reliable way of providing monolithically integrated vertical cavity surface-emitting laser (VCSEL) arrays is still needed.
In a vertical cavity surface-emitting laser (VCSEL), the lasing wavelength may be determined by the length of a Fabry-Perot cavity formed by two distributed Bragg reflectors (DBRs) separated by the semiconductor optical cavity active region that includes layers with optical gain. The optical gain in a vertical cavity surface-emitting laser (VCSEL) is typically provided by quantum wells. Each quantum well has a gain spectrum with a single peak wavelength, and some spectral width over which gain is present. Each distributed Bragg reflector (DBR) is composed of quarter-wave layers of alternating high and low refractive indices. The distributed Bragg reflector (DBR) reflectivity is characterized by a complex amplitude and phase spectrum. The amplitude spectrum exhibits a high reflectivity region at the center of which the reflectivity is highest. The width of the high reflectivity region is referred to as the distributed Bragg reflector (DBR) stop-band width. The phase characteristic of the distributed Bragg reflector (DBR) varies approximately linearly over the stop-band width. The lasing wavelength of a vertical cavity surface-emitting laser (VCSEL) is determined by the optical length of the semiconductor cavity and the phase characteristics of the distributed Bragg reflectors (DBRs). The gain provided by the active layer, necessary to achieve lasing (threshold gain) is determined by the roundtrip cavity loss that includes material absorption and the distributed Bragg reflector (DBR) transmission. A monolithic multiple-wavelength vertical cavity surface-emitting laser (VCSEL) array requires side-by-side fabrication of vertical cavity surface-emitting lasers (VCSELs) with varying lasing wavelengths, but otherwise uniform laser characteristics, such as threshold gain and current, and efficiency. This implies that the vertical structure of the lasers must vary from device to device on the same wafer, while the cavity losses, material gain, and the distributed Bragg reflector (DBR) transmission remain largely unchanged. The lasing wavelength variation is most commonly realized by changing the optical length of the semiconductor cavity.
One conventional method of making a monolithic multiple wavelength vertical cavity surface-emitting laser (VCSEL) array uses non-uniform growth due to a thermal gradient. The backside of a substrate is patterned prior to epitaxial growth in a molecular beam epitaxy (MBE) reactor. The resulting backside pattern produces a thermal gradient on the surface of the substrate when the wafer is heated. Because growth rate is temperature dependent, there is a variable material thickness and, hence, a variable laser wavelength along the thermal gradient. One disadvantage of this conventional approach is the fact that the arrays are limited to linear geometries. To date, it has been difficult to control the wavelengths precisely and repeatedly over large areas of the wafer.
Another conventional method is to grow a partial vertical cavity surface-emitting laser (VCSEL) structure including the lower distributed Bragg reflector (DBR), the active region, and some part of the upper distributed Bragg reflector (DBR). The wafer is masked and anodically oxidized to some controlled oxide thickness over the exposed portions. A selective etch is then used to remove the oxide. This process is repeated to create different effective cavity lengths for each laser in an array. The remainder of the vertical cavity surface-emitting laser (VCSEL) structure is regrown over the patterned wafer. However, each selective etch is sensitive to voltage and concentration variations that may affect the depth, resulting in reduced control over wavelength spacing between devices in the array.
Yet another conventional method of making a monolithic multiple wavelength vertical cavity surface-emitting laser (VCSEL) array is described, for example, in U.S. Pat. No. 6,117,699 to Lemoff et al. (xe2x80x9cthe Lemoff et al. ""699 patentxe2x80x9d), describing an array of N-wavelength vertical cavity surface-emitting lasers (VCSELs) that can be grown with wavelength control. First, as shown in FIG. 1, a foundation vertical cavity surface-emitting laser (VCSEL) structure 100 is grown on a gallium arsenide (GaAs) substrate 105. The foundation vertical cavity surface-emitting laser (VCSEL) structure 100 includes a lower distributed Bragg reflector (DBR) 110 in an optical cavity 145. The lower distributed Bragg reflector (DBR) 110 includes M pairs of layers 115, 120, 125, 130, 135 and 140 (M=6, in FIG. 1), each member of each pair having an index of refraction differing from the other member of each pair. For example, the lower member 115a of the pair 115 may comprise aluminum arsenide (AlAs) and the upper member 115b of the pair 115 may comprise aluminum gallium arsenide (AlxGalxe2x88x92xAs, where 0.15 less than x less than 1).
The optical cavity 145 also includes a first intrinsic (non-doped) layer 150, an optical gain layer 155 and a second intrinsic (non-doped) layer 160. The optical cavity 145 also includes N-paired semiconductor phase shift epitaxially grown layers 165, 170, 175 and 180 (N=4 in FIG. 1) of aluminum gallium arsenide (AlGaAs) and indium gallium phosphide (InGaP), where N is the desired number of different wavelengths.
Next, a region of one of the N-paired semiconductor phase shift layers is lithographically patterned (masked and etched). For example, as shown in FIG. 1, a mask 185 is formed and portions 190 and 195 (shown in phantom) of the paired semiconductor phase shift epitaxially grown layer 180 are removed by selective etching. The steps of patterning (masking and etching) are repeated for a total of at least (Nxe2x88x921)/2 patterning (masking and etching) steps until regions of at least Nxe2x88x921 of the N-paired semiconductor phase shift layers are etched. For example, as shown in FIG. 2, a mask 200 is formed and portions 205 and 210 (shown in phantom) of the paired semiconductor phase shift epitaxially grown layers 170, 175 and 180 are removed by selective etching, forming 4 different effective optical cavity lengths corresponding to 4 different lasing wavelengths.
Finally, an upper vertical cavity surface-emitting laser (VCSEL) structure (not shown) is grown. The upper vertical cavity surface-emitting laser (VCSEL) structure may include an upper distributed Bragg reflector (DBR) similar to the lower distributed Bragg reflector (DBR) 110, but typically having fewer than M pairs of layers, each member of each pair having an index of refraction differing from the other member of each pair. The upper distributed Bragg reflector (DBR) typically has fewer than M pairs of layers so that the reflectivity of the upper distributed Bragg reflector (DBR) is less than the reflectivity of the lower distributed Bragg reflector (DBR) 110. Equivalently, the transmissivity of the upper distributed Bragg reflector (DBR) is greater than the transmissivity of the lower distributed Bragg reflector (DBR) 110. Consequently, more laser photons are emitted from the upper surface (not shown) of the vertical cavity surface-emitting laser (VCSEL) than into the substrate 105.
However, this Lemoff et al. ""699 patent method necessarily requires at least (N-1)/2 masking steps, to produce an array of N-wavelength vertical cavity surface-emitting lasers (VCSELs). For a large number N of wavelengths, such methods may become expensive, unwieldy, impractical and inefficient and may suffer from a loss of yield.
For example, as shown in FIG. 3, a foundation vertical cavity surface-emitting laser (VCSEL) structure 300 is grown on the gallium arsenide (GaAs) substrate 105. The foundation vertical cavity surface-emitting laser (VCSEL) structure 300 includes the lower distributed Bragg reflector (DBR) 110 and an optical cavity 345. The optical cavity 345 includes the first intrinsic (non-doped) layer 150, the optical gain layer 155 and the second intrinsic (non-doped) layer 160. The optical cavity 345 also includes N-paired semiconductor phase shift epitaxially grown layers 165, 170, 175, 180, 365, 370, 375 and 380 (N=8 in FIG. 3) of aluminum gallium arsenide (AlGaAs) and indium gallium phosphide (InGaP), where N is the desired number of different wavelengths.
Next, a region of one of the N-paired semiconductor phase shift layers is lithographically patterned (masked and etched). For example, as shown in FIG. 3, a mask 305 is formed and portions 390 and 395 (shown in phantom) of the paired semiconductor phase shift epitaxially grown layer 380 are removed by selective etching. The steps of patterning (masking and etching) are repeated for a total of at least (Nxe2x88x921)/2 patterning (masking and etching) steps until regions of at least Nxe2x88x921 of the N-paired semiconductor phase shift layers are etched. For example, as shown in FIG. 4, a mask 400 is formed and portions 405 and 410 (shown in phantom) of the paired semiconductor phase shift epitaxially grown layers 370, 375 and 380 are removed by selective etching. Then, as shown in FIG. 5, a mask 500 is formed and a portion 505 (shown in phantom) of the paired semiconductor phase shift epitaxially grown layer 365 is removed by selective etching. Finally, two more patterning steps (similar to those shown in FIGS. 1 and 2) may be performed to remove respective portions (similar to the portions 190, 195, 205 and 210 shown in FIGS. 1 and 2) from the paired semiconductor phase shift epitaxially grown layers 170, 175 and 180 by selective etching, forming 8 different effective optical cavity lengths corresponding to 8 different lasing wavelengths, but using as many as 5 patterning steps.
Resonant optical cavities are used in many devices including vertical cavity surface-emitting lasers (VCSELs), optical filters (such as band-pass filters), resonant cavity-based optical devices, resonant cavity-based opto-electronic devices and resonant cavity-enhanced photodetectors. The general structure of such a device consists of a cavity region including a reflector on each end of the device. These reflectors are often distributed Bragg reflectors (DBRs) that are made from multiple pairs of material with alternating refractive indices. The higher the contrast in material index of refraction, the lower the number of mirror pairs that are required to achieve the desired reflectivity. For many structures these layers are made from semiconductors whose refractive indices tend to be similar. It is also possible, and in some cases desirable, to fabricate the top reflector using dielectric materials with a greater contrast than is available using distributed Bragg reflectors (DBRs) made entirely of semiconducting materials.
There are also applications, in optical communication for instance, where it is desirable to have arrays of lasers, filters, or detectors with closely-spaced wavelengths. The wavelength may be changed by altering the effective cavity length of the device. The effective cavity length at a given wavelength is determined by the layer thicknesses and material properties in the optical cavity and the phase shift due to the reflectors. There are previous approaches (U.S. Pat. No. 6,117,699) in which the optical cavity length is adjusted by varying the thickness of material within the optical cavity. This can be achieved by depositing and selectively removing layer thicknesses within the optical cavity. This approach relies on the epitaxial growth of a specific pair of semiconductor materials to achieve the precision and etch selectivity required. However, this approach limits the technique to a specific range of wavelengths.
The sensitivity of the resonant (for example, the lasing) wavelength depends in part on the distributed Bragg reflector (DBR) materials utilized. However, the choice of distributed Bragg reflector (DBR) materials is often dictated by other concerns (compatibility with device operation, compatibility with the processing, reliability, and the like). Since the required sensitivity depends on the desired wavelength spacing for the particular application, it would be desirable to be able to adjust the sensitivity of the optical cavity to the thickness of the tuning layers. In particular, for improved process control, it would often be desirable to decrease the sensitivity of the wavelength to variations in the tuning layer thickness.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided, the method comprising forming an optical cavity for an optical device and forming at least one reflector for the optical cavity for the optical device, the at least one reflector having at least two sections. The method also comprises providing at least one of a tuning layer between the at least two sections of the at least one reflector and different refractive index contrasts for the at least two sections of the at least one reflector.
In another aspect of the present invention, a device is provided, the device comprising an optical cavity for an optical device, the optical cavity having at least one reflector having at least two sections. The device also comprises at least one of a tuning layer between the at least two sections of the at least one reflector and different refractive index contrasts for the at least two sections of the at least one reflector.